A variety of implants are known in the art for attaching an elongated member, e.g. an elongated bar or rod, to one or more bones for the purpose of correcting deformity, promoting healing, or other therapeutic uses. Among such devices are those disclosed in U.S. Pat. No. 5,005,562 to Cotrel, U.S. Pat. No. 5,797,911 to Sherman et al., and U.S. Pat. No. 6,280,442 to Barker et al. In those devices, a U-shaped head is provided for accommodating an orthopedic rod, such as that used in corrective spinal surgery. A threaded element or hook connects the U-shaped head to a bone portion, and a set screw locks the elongated rod within the U-shaped head.